1. Field of the Invention
The present invention relates to a method of drawing an optical fiber preform under heat to manufacture an optical fiber.
2. Description of the Related Art
In the manufacture of, for example, a quartz series optical fiber, an optical fiber preform having an outer diameter of 40 to 100 mm is inserted from above into a heating furnace. A tip portion of the preform is heated at about 2,000.degree. C. within the furnace so as to be melted and, then, drawn into an optical fiber having a predetermined outer diameter. It is important to control the drawing operation so as to prevent the drawn optical fiber from being changed in properties with progress of the drawing operation. In other words, the drawn optical fiber is required to exhibit uniform properties throughout the entire lengthwise region.
It was customary in the past to set the heating temperature within the furnace to achieve the optimum properties of the optical fiber formed about the start-up time of the drawing operation. Then, the heating temperature within the furnace is set constant during the succeeding drawing operation by monitoring the temperature within the furnace with, for example, a radiation thermometer.
A longer optical cable is required nowadays. Naturally, an optical fiber preform is rendered bulky and larger in outer diameter. If the temperature within the heating furnace is set to achieve the optimum properties of the optical fiber formed about the start-up time of the drawing operation, followed by keeping the furnace temperature constant, as in the conventional method referred to above, a change takes place in the amount of heat actually imparted to the optical fiber preform. Specifically, the heat amount imparted to the optical fiber preform is increased with progress of the drawing operation. In other words, the temperature of the optical fiber preform is gradually elevated with progress of the drawing operation. As a result, the drawing operation is rendered nonuniform so as to bring about nonuniform transmitting characteristics, particularly transmission loss, of the drawn optical fiber throughout the entire lengthwise region of the optical fiber. The change in the heat amount noted above is considered to be derived from a change in the amount of heat radiation from the optical fiber preform itself, said change taking place with progress of the drawing operation applied to the optical fiber preform inserted into the heating furnace.